It has been found advantageous to inspect manufactured parts such as jet engine turbine blades by passing penetrating radiation such as x-rays through those inspected parts using equipment as described and claimed in U.S. Pat. No. 4,835,394, the parent of the present case, which is hereby incorporated herein by reference. The intensity of the penetrating radiation after having passed through a part indicates the nature of the part and may be used to create an image of the part so that any flaws or defects may be observed.
Reliable observation of flaws in parts such as turbine blades requires a high degree of resolution in any detector of penetrating radiation used in such inspection systems. To achieve high resolution, an ionization detector in the form of a sealed chamber containing closely spaced detector elements and pressurized dielectric such as xenon gas is used. Radiation that has passed through the part being inspected is admitted to the chamber through a radiation permeable window. The radiation admitted to the chamber ionizes the dielectric to an extent related to its intensity. The charge created by that ionization is collected upon the detector elements and transmitted to charge storage circuitry outside the ionization detector, which may be a charging circuit comprising a resistor in series with a capacitor. Part of the circuitry involved in transmitting such charge may include a flexible electric cable connecting the ionization detector with the charge storage circuitry. Because many detector elements must be connected with charge storage circuitry, this cable is most conveniently in the form of one or more ribbon cables each containing a row of closely spaced parallel conductors. Each of the conductors is electrically connected to a detector element and to a resistor in series with a capacitor.
The amount of charge created in such ionization detectors is very small. The current resulting from transfer of that amount of charge to the storage circuitry is on the order of picoamperes. The circuitry associated with the collection and storage of such small amounts of charge is particularly prone to phenomena which may adversely impact the accurate transmission of charge from the ionization detector to the charge storage circuitry. Thus, it is particularly difficult to ascertain the true amount of penetrating radiation entering the ionization detector which makes it difficult to produce accurate images of a part being inspected.
Factors which have been found to impair the accurate transmission of charge created in the ionization detector are numerous and difficult to eliminate. Those factors include electromagnetic interference such as that produced by fluorescent lighting in the vicinity of the inspection apparatus. They also include problems caused by electrostatic charge build up and discharge in the vicinity of the inspection apparatus. To give some idea of the magnitude of this electrostatic charge based problem, merely combing one's hair in close proximity to the transmission system (e.g. 5-10 feet away) influences the charge transfer process. Clothing, such as polyester clothing, which tends to pick up an electrical charge, likewise influences the charge transfer process when worn by people in the same vicinity of the inspection apparatus. An additional difficulty in accurately transferring charge from the ionization detector is caused by mechanical vibration in the vicinity of the circuitry through which the charge flows. Even breathing near or walking past the inspection apparatus has a noticeable effect.
None of these phenomena can be entirely eliminates from the environment of the inspection equipment. Therefore, efforts must be made to increase the immunity of the circuitry involved in charge transfer to the effects of those phenomena. Any significant length of flexible cable used for transferring charge is particularly susceptible to the ill effects of the phenomena described above. Accordingly, ways of increasing the immunity of the cable to the effects of phenomena which may distort signals from the ionization detector that flow through the cable are desirable.
One way to try to avoid the problems with using flexible cable is to shield each of the conductors in the cable. Such a cable in the form of a ribbon cable having a number of individually shielded conductors is available commercially and was tried as a solution to those problems. However, it was found that shielding only the individual conductors in a ribbon cable is insufficient. The next thing that was tried was to wrap an additional conductive shield around the entire shielded cable. It was found that this arrangement helped to reduce the electrostatically based interference, but the electromagnetically based interference and the vibrationally induced interference were still unacceptable. An additional conductive shield was then wrapped around the first shield. This additional shield reduced the electromagnetic interference to acceptable levels, but the inspection apparatus was still subject to vibrational effects. It was only after the first shield was adhered to the cable and the second shied was adhered to the first shield in accordance with the invention of this application that the effects of the vibrational problems were also reduced to acceptable levels.
The invention of this application thus reduces the effects of external interference on signals carried by electric cables. As is apparent from the description below, this may be accomplished in a simple and inexpensive manner through straightforward assembly of readily available components. The invention of this application is useful in any situation where interference with accurate signal transmission is a concern. It is particularly useful in situations involving low level signals, such as those encountered in the transmission of signals from ionization detectors used in high resolution inspection apparatus.